The circumstances of Abu Muhammad al-Adnani's death in Aleppo, Syria, have not been confirmed. Officials in Washington — who had offered a $5 million reward for al-Adnani, the terror group's No. 2 man — had no immediate comment.

The 37-year-old, who was ISIS' director of external operations and main spokesman, is best known for issuing an edict for lone wolves to kill Westerners in September 2014.

"If you can kill a disbelieving American or European — especially the spiteful and filthy French — or an Australian, or a Canadian, or any other disbeliever from the disbelievers waging war, including the citizens of the countries that entered into a coalition against the Islamic State, then rely upon Allah, and kill him in any manner or way however it may be," he said.

"Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him, or poison him."

As NBC News reported in December, al-Adnani was the No. 1 name on the U.S. list of ISIS leaders it wanted to kill.

"Adnani has been the main voice behind issuing ISIS threats to the West," Laith al-Khouri of Flashpoint Intelligence, an NBC counterterrorism analyst, said at the time. "He is so admired and glorified by jihadists worldwide that he stands as a primary point of recruitment."

Al-Adnani was one of the first foreign fighters to oppose U.S. Coalition Forces in Iraq after he crossed the border from his native Syria in 2003.

He swore allegiance to Abu Musab al-Zarqawi, the Al Qaeda in Iraq leader later killed by U.S. fighter-bombers. He reportedly was captured in 2005 and taken into custody at a camp run by the U.S. military, but was freed in 2010.

After his release, he became the chief propagandist for ISIS, and by 2014 he had assumed a top operational role. The U.S. declared him a "specially designated global terrorist" in August 2014.